The present invention relates generally to retransmission protocols for wireless communication systems and, more particularly, to the selection of retransmission parameters for hybrid automatic repeat request operations in wireless communication systems.
High-Speed Downlink Packet Access (HSDPA) for Wideband Code Division Multiple Access (WCDMA) and Long Term Evolution (LTE) networks use Hybrid Automatic Repeat Request (HARQ) at the physical layer to mitigate errors that occur during transmission of data. In HARQ, error detection bits or check bits are added to information bits to be transmitted. The information bits with the added error detection bits are then coded using a forward error correction code to obtain a block of coded data bits. The transmitter transmits a portion of these coded bits to the receiver in an initial transmission. The receiver decodes the received bits and uses the error detection bits to check for uncorrected errors. If the received data block is successfully decoded, the receiver sends a positive acknowledgement (ACK) to the transmitter over a reverse control channel. If the received data block is not correctly decoded, the receiver can request a retransmission by sending a negative acknowledgement (NACK) to the transmitter over a reverse control channel.
In conventional HARQ operations, a single acknowledgement bit is sent from the receiving terminal to the transmitting terminal to indicate whether the transmitted data packet was correctly decoded. Typically, a “1” is transmitted to indicate successful decoding and a “0” is sent to indicate a decoding failure and to request a retransmission. The ACK/NACK informs the base station whether the data packet was correctly received by the user terminal. If the data packet is correctly received by the user terminal, the base station can proceed with the transmission of new data packets. In the event that the data packet is not correctly received by the user terminal, the base station may either repeat the original transmission or send additional coded data bits, which may be combined with the previously transmitted data bits prior to decoding. Sending additional coded data bits lowers the effective coding rate and increases the probability that the decoder will successfully decode the coded data bits.
One drawback of conventional HARQ is that the state of the decoder is not considered in determining the parameters of the retransmission. If the decoder is close to finding on a solution, it may need only a small amount of additional data to successfully decode the transmitted data. On the other hand, if the decoder is still far from finding a solution, then more data may be needed. In the first case, the transmitter may send more data in the retransmission than is needed by the decoder, which wastes resources and creates unnecessary interference. In the second case, the transmitter may not send enough data in the next retransmission, which will result in increased delays.